Stability of AuCl2- from 25 to 1000 °C at Pressures to 5000 bar and Consequences for Hydrothermal Gold Mobilization

Gold is transported in high-temperature chloride-bearing hydrothermal fluids in the form of AuCl2-. The stability of this complex has been extensively studied, but there is still considerable disagreement between available experimental data on the temperature region 300-500 degrees C. To solve this problem, we measured the solubility of gold in HCl/NaCl fluids (NaCl concentration varied from 0.1 to 3 mol.(kg.H2O)(-1)) at 450 degrees C and pressures from 500 to 1500 bar (1 bar = 10(5) Pa). The experiments were performed using a batch autoclave method at contrasting redox conditions: in reduced experiments hydrogen was added to the autoclave, and in oxidized experiments the redox state was controlled by the aqueous SO2/SO3 buffer. Hydrogen pressure in the autoclaves was measured after the experiments in the reduced system. The gold solubility constant, Au-(cr) + HCl degrees((aq)) + Cl- = AuCl2- + 0.5 H-2 degrees((aq)), was determined for the experimental T-P parameters as log K-s degrees = -4.77 +/- 0.07 (500 bar), -5.11 +/- 0.08 (1000 bar), and -5.43 +/- 0.09 (1500 bar). These data, together with values from the literature for temperatures from 25 to 1000 degrees C, were fitted to the simple equation log Ks degrees = 4.302-7304.T(K)(-1) - 4.77.log d(w) + 11080.(log d(w)).T(K)(-1) -6.94 x 10(6).(log d(w)) T(K)(-2), where d(w) is the pure water density. This equation can be used together with the extended Debye-Huckel equation for activity coefficients to calculate gold solubility at pressures up to 5000 bar at fluid chlorinities at least up to 30 wt %. The speciation of gold in natural chloride-bearing fluids is discussed.